Add plugin "exercises"

Author: SamyPesse

book.json

@@ -0,0 +1,5 @@
+{
+    "plugins": [
+        "exercises"
+    ]
+}

package.json

@@ -0,0 +1,7 @@
+{
+    "name": "book-javascript",
+    "version": "0.0.0",
+    "dependencies": {
+        "gitbook-plugin-exercises": "*"
+    }
+}